Monsoons

Most of the year, some places get very little rain. Then the wind changes direction.

Now it blows in from the warm sea. It brings big, dark clouds. Then the rain pours down, day after day. This is the monsoon.

Imagine a place where it barely rains for months. The ground is dusty, and people are waiting. Then the wind turns around. Warm, damp air rolls in from the ocean, dark clouds pile up and heavy rain begins. This is the monsoon.

National Geographic defines a monsoon as a seasonal change in the direction of a region's prevailing, or strongest, winds. Monsoons cause wet and dry seasons across much of the tropics. They are most famous around the Indian Ocean, where the summer monsoon and the winter monsoon shape the climate of most of India and Southeast Asia. The word monsoon is used both for the wind and for the rainy season it brings.

A monsoon is defined by wind, not rain: National Geographic describes it as a seasonal change in the direction of a region's prevailing winds, producing wet and dry seasons throughout much of the tropics. The best-known system is South Asia's, where a rainy summer monsoon from the southwest alternates with a drier winter monsoon from the northeast.

The Asian-Australian monsoon system is vast, stretching from northern Australia to Russia's Pacific coast and across the Indian Ocean, and similar seasonal wind reversals occur in other regions, including North America. The India Meteorological Department (IMD) tracks the South Asian summer monsoon day by day: its arrival, or onset, its advance across the country, its active and break phases, and its withdrawal, because each stage affects millions of farmers, water managers and city planners.

In spring and summer, the Sun heats the land a lot. The land gets very hot.

The hot air over the land pulls in wet air from the sea. The wet air makes clouds and rain. In winter, the wind blows the other way, from the land. Then it is dry.

The monsoon starts with the Sun. As summer comes to the Northern Hemisphere, the land around the Arabian Sea heats up strongly. The heat makes an area of low air pressure form over the land. The IMD explains that a strong heat low acts like a suction device, pulling in moist air.

So, in the summer monsoon season, warm, moist air from the southwest Indian Ocean blows toward India, Sri Lanka, Bangladesh and Myanmar. When the wet wind reaches the Western Ghats, a long line of mountains along India's west coast, heavy rain falls along the coast. In winter the pattern flips. The dry winter monsoon blows from the northeast, from high above Mongolia and northwestern China, and lasts from about October to April.

The IMD explains the driving engine. As the Sun moves north in the Northern Hemisphere, the lands around the Arabian Sea receive large amounts of heat, from sunlight and from the hot ground, about 160 W/m² of surface heat flux in June over the arid zones of northwest India, Pakistan and the Middle East. A shallow heat low forms; an intense one acts as a suction device for moist air along the monsoon trough, an elongated low-pressure area from Pakistan to the head of the Bay of Bengal. Monsoon lows and depressions, many forming over the Bay of Bengal and moving west, are the principal rain-bearing systems; on average about two depressions form in each month from June to September.

Topography concentrates the rain. The Western Ghats, about 1,000 km long and 200 km wide, block the southwesterly winds; IMD notes that offshore vortices formed when winds are deflected around them bring heavy to very heavy rain to the west coast. The winter monsoon, from about October to April, blows from the northeast; National Geographic notes it is weaker partly because the Himalayas block much of its wind and cold air.

The monsoon rain reaches India in early summer. It comes to Kerala, in the south, first.

Then it moves north, bit by bit. Soon the whole country gets rain. In autumn, the monsoon goes away again.

People in India watch closely for the monsoon's arrival, called its onset. It usually reaches Kerala, at the southern tip of India, first. Then it spreads north across the country over the next few weeks, and in autumn it withdraws.

In 2025, the IMD reported that the monsoon set in over Kerala on 24 May, 8 days earlier than normal. It covered the whole country by 27 June, 11 days earlier than its normal date of 8 July, and it had withdrawn from the entire country by 16 October. That year's total monsoon rain for India was 108 percent of the long-term average, a little more than usual.

The IMD treats onset over Kerala as the monsoon's formal arrival on the Indian mainland and tracks its advance and withdrawal against normal dates. Its report on 2025 records an unusually early season: the monsoon reached the south Andaman Sea on 13 May, 9 days ahead of normal; set in over Kerala on 24 May, 8 days early; covered the entire country by 27 June, 11 days ahead of its normal date of 8 July; began withdrawing from west Rajasthan on 14 September; and withdrew from the whole country on 16 October.

Seasonal totals are judged against a long-period average (LPA). For the country as a whole, June to September 2025 rainfall was 108 percent of LPA (the report's preface calls this normal, within 10 percent of the LPA, while IMD's forecast categories class 104 to 110 percent as above normal), with every month above average (June 109, July 105, August 105 and September 115 percent). The regions differed sharply: Northwest India received 127 percent and Central India 115, but East and Northeast India only 80. Nineteen low-pressure systems formed during the season, including five depressions and two deep depressions.

Mumbai is a big city in India. From January to April, it hardly rains at all.

Then the monsoon comes. In July, it rains and rains! Most of Mumbai's rain for the whole year falls in just four months.

WMO figures for Mumbai, on India's west coast, show how strong the monsoon is. From January to May, very little rain falls: in March, on average, only about 0.1 millimeters. Then the monsoon arrives. In June, about 504 millimeters fall, and in July, the wettest month, about 819 millimeters.

In an average year, Mumbai gets about 2,431 millimeters of rain, and about nine-tenths of it falls from June to September. National Geographic notes that people in Mumbai are used to streets flooding with almost half a meter of water every summer, and very strong monsoons can cause much worse floods.

WMO normals for Mumbai (1951–2000) show the monsoon's dominance: monthly means of 15.1 mm in January, 1.0 in February, 0.1 in March and 0.5 in April, then 504.2 mm in June, 819.4 in July, 546.8 in August and 325.2 in September, out of an annual total of about 2,431 mm. June to September supply about 2,196 mm, roughly 90 percent of the year's rain.

That concentration brings both water and risk. National Geographic notes that Mumbai's residents are used to streets flooding with almost half a meter of water each summer, and that on July 26, 2005, the city received almost a meter (39.1 inches) of rain in a single day. Dhaka shows a similar rhythm farther east: its WMO normals (1971–2000) climb from 7 mm in January to 373 mm in July, with rain on 21 days in both July and August.

Farmers need monsoon rain to grow rice and tea. The rain fills wells and rivers.

The water also helps make electricity. When the rain is late, farmers and towns have a hard time.

The IMD calls the monsoon rain crucial for India. The IMD says about 75 percent of India's yearly rain falls in the monsoon season, and farming, which employs nearly half of India's people, depends on it. Rice and tea are among the crops that need the summer monsoon. Many places do not have big irrigation systems, and the summer rain fills wells and underground water supplies for the rest of the year.

Monsoon rain also fills reservoirs behind dams that make hydroelectric power. When the monsoon is late or weak, harvests can suffer and electricity can cost more. The summer monsoon has even been called India's true finance minister.

The IMD's 2025 report opens with the monsoon's stakes: agriculture employs nearly half of India's population; about 75 percent of annual rainfall arrives in the monsoon season, replenishing rivers, lakes and groundwater for irrigation and drinking water; it drives hydropower; and excess or deficient rain can bring floods or droughts that affect food security and livelihoods.

National Geographic adds that many areas lack large irrigation systems and have shallow aquifers, so the summer monsoon refills wells for the rest of the year; that rice and tea depend on it; and that a late or weak monsoon can reduce harvests, raise electricity costs and force food imports, which is why it has been called India's true finance minister. Because so much depends on timing, the IMD issues seasonal, monthly and extended-range forecasts, and runs agro-meteorological services for farmers.

There are monsoons in other places too.

There is one in parts of Australia and Southeast Asia. There is even one in the southwest of the United States and in Mexico. It brings summer rain to the desert.

South Asia's monsoon is the most famous, but it is not the only one. The Asian-Australian monsoon system stretches from northern Australia to Russia's Pacific coast. In some places, such as Indonesia and Malaysia, the winter monsoon is the rainy one, because it brings moist air from the South China Sea. Singapore has two monsoon seasons each year.

North America has a monsoon too. Each summer, moist air from the Gulf of California and the Gulf of Mexico meets over the mountains of central Mexico and then moves north into Arizona, New Mexico and Texas. National Geographic says it is the main source of water for most desert ecosystems in that region.

Monsoon winds exist in other parts of the world too. National Geographic describes the Asian-Australian monsoon spanning northern Australia to Russia's Pacific coast; unlike western Southeast Asia, the eastern, Pacific-facing coasts have their rainy season in winter, when the monsoon carries moist air from the South China Sea to places such as Indonesia and Malaysia. The Meteorological Service Singapore identifies a Northeast Monsoon from December to early March and a Southwest Monsoon from June to September, separated by inter-monsoon periods.

The North American monsoon arrives once a year, usually in mid-summer, as moist air from the Gulf of California and the Gulf of Mexico converges over the Sierra Madre Occidental before reaching Arizona, New Mexico and Texas. It is the primary water source for most desert ecosystems there and can help firefighters, though heavy rain can disrupt daily life. National Geographic also links the global picture to the Intertropical Convergence Zone, sometimes called the Monsoon Zone.